It has been suggested that specific microorganisms are important in the pathogenesis of early-onset periodontitis. However, this is still a controversial issue since most published data are based on cross- sectional studies and there are only few longitudinal studies of early-onset periodontitis. In this project 248 young individuals with and without disease are followed-up for 6 years with clinical measurements of periodontal parameters, and subgingival plaque is collected from all subjects. Presence and amount of putative periodontal pathogens in the dental plaque will be analyzed using DNA probes. In addition, plaque from 20 subjects with the most severe form of the disease will be analyzed by standard culture techniques. The association between these microorganisms and the disease progression during 6 years will be studied and the role of specific bacterial species in the pathogenesis of early-onset periodontitis will be assessed.